<aura:attribute name="cbasisshow" type="boolean" default="false">
</aura:attribute

  <div class="row">
      <lightning:input type="toggle"
                     label="Annual Values Contain Cost Basis?"
                     name="CB?"
                     messageToggleActive="Yes"
                     messageToggleInactive="No"
                     onchange="{! c.showCostBasis}"
                     aura:id="basis"/>

      <aura:if isTrue="{!v.cbasisshow}">
      <lightning:input type="number"
                       name="Cost Basis"
                       aura:id= "basis"
                       label="Cost Basis"
                       value="12345"/>
     </aura:if>
   </div>

showCostBasis : function(component, event, helper) {
    var isChecked = component.find('basis').get('v.checked');
    console.log(isChecked);
      if(isChecked) {
        component.set('v.cbasisshow', 'true')
      } else {
          component.set('v.cbasisshow', 'false');
        }
}